When Serena found the list of people her mother Lily had slept with, she said with exasperation, ""Mom. Slash? Really?"
Slash (AKA Saul Hudson) is a musician best known for his connection to Guns 'n' Roses. But he does have a televersion, albeit one with a strange pedigree. He appeared in an episode of 'The Drew Carey Show' and then apparently he crossed over to the Tooniverse. First he made contact with that animated dimension in an episode of 'Space Ghost: Coast To Coast', and then appeared in an episode of 'South Park' and several episodes of 'Kid Notorious', fully animated himself.
I suppose one could consider his appearance in the Kentucky Derby episode of Anna Nicole Smith's reality show to be another notch in his belt for Toobworld appearances, but that's just too sad to think about......
So Slash is just as much a TV character as Serena and Lily, and therefore - like Larry Sanders with Roseanne and Ellen DeGeneres (but not together!) - capable of having sex with Lily.